Traditionally, tilt switches made from mercury capsules have been used in a wide variety of applications. For instance, mercury switches have been utilized in lighting circuits that control lights located on the underside of vehicle hoods. When the hood is opened, and hence tilted beyond a certain angle, the switch would turn on a light. A non-automotive example is in simple residential thermostats where the mercury switch is mounted on a spiral flat spring and used to switch on and off a HVAC system as the spring thermally expands or contracts. However, in an effort to limit mercury contamination due to improper disposal of the mercury switches at the end of their useful life, it has become desirable to find a tilt switch design which does not utilize mercury. This has resulted in the development of different tilt switch designs that use non-mercury based components, such as permanent magnet/reed switch combinations, to sense changes in inclination.
For example, U.S. Pat. No. 4,820,888 issued Apr. 11, 1989 to Shields discloses the use of a magnetically operated tilt switch assembly that generally includes a reed switch, an elongated cavity, and a sliding magnet. When the switch assembly tilts beyond a predetermined angle, the magnet slides within the cavity towards and away from the reed switch, which is located at one axial end of the cavity. The proximity of the magnet to the reed switch controls the operation of the switch assembly.
Similarly, U.S. Pat. No. 3,564,171 issued Feb. 16, 1971 to Hammond discloses a magnetic filed activated tilt switch assembly having an elongated cavity for receiving a sliding magnet. The magnet comprises a bar magnet sandwiched between two ferrous balls which are wide enough that the magnet does not come in direct contact with the sides of the cavity. As the switch housing is tilted beyond a predetermined angle, the magnet slides towards or away from a reed switch mounted on the topside of the cavity, thereby closing the switch when the magnet is in close proximity and opening the switch when the magnet is at the other end of the cavity.
U.S. Pat. No. 3,601,729 issued Aug. 24, 1971 to Hierta also teaches the use of a tilt switch assembly incorporating a reed switch to detect inclination of the device beyond a certain angle. The tilt switch assembly that includes a switch housing having an elongated V-shaped cavity which contains a cylindrical magnetic member. The apex of the cavity is closest in distance to a reed style switch. Accordingly, when the switch housing is in an upright (non-tilted) orientation, the magnetic flux from the magnetic member is strong enough to pull the two contacts of the reed switch together, thereby closing the switch. As the switch housing is tilted, the magnetic member rolls away from the apex, thereby becoming sufficiently spaced from the reed switch and unable to pull the switch contacts together.
U.S. Pat. No. 5,209,343 issued May 11, 1993 to Romano et al. discloses a magnetic field sensing tilt switch, shown in FIG. 21, that generally comprises a cup shaped housing, a magnetized ball, and a magnetic sensor switch. As the switch assembly is tilted, the ball rolls towards and away from the magnetic field sensing switch, thus causing the switch to change states according to the position of the ball within the housing.
Further examples of tilt switch assemblies utilizing reed switches can be found in U.S. Pat. Nos. 5,256,839, 5,477,428, 5,669,696, and 5,798,912. Other magnetically-actuated switch designs have been proposed using Hall effect sensors rather than reed switches. See, for example, U.S. Pat. No. 5,373,125, issued Dec. 13, 1994 to Ford et al. This patent discloses a magnetic field sensing tilt switch assembly wherein a pivotal member having a permanent magnet pivots within a switch housing containing a Hall effect sensor. When the switch assembly is in an upright position the magnetic field is aligned with the sensor, however, when the switch assembly is tilted, the magnet and the sensor become unaligned, which results in the switch changing states.
Apart from magnetic switches, other non-mercury based technologies such as photoelectric detectors have been used in tilt switches. For example, U.S. Pat. No. 5,202,559 utilizes a photo-optical interrupter and a box-like chamber and ball to detect a tilt beyond a predetermined angle. Two of the four interior side walls of the box have light emitting means mounted such that they communicate with light detecting means mounted on the opposing interior wall, and each interior side wall has a semi-spherical recess designed to receive the ball. When the switch is tilted beyond a predetermined angle, the ball rolls downwards and seats on the lowest recess, thereby obstructing the light emitting and detecting means associated with that recess and changing the state of the switch.
Likewise, U.S. Pat. No. 5,209,343 discloses a photo-optical tilt switch, seen in FIG. 18, that includes a cylindrical housing containing a weighted ball. At one axial end of the housing is a light transmitter and light receiver mounted in opposing fashion along the interior walls of the housing. Thus, when the switch assembly is tilted, the ball rolls to the end of the cylindrical housing having the light sensing components and prevents the optical receiver from receiving the emitted light, thus causing the switch to change states. Additional tilt switch designs incorporating photo-optical interrupters can be seen in U.S. Pat. Nos. 5,373,153, 6,011,254, and 6,140,635.